I've checked the object model for the Document.SaveAs StructSaveAsOptions object and it doesn't appear to have any such parameter. I know I can't be the only person who needs to do silent saves / prints so has anyone else come across this same issue?

Have you tried this yourself on X5? I'm concerned that the functional behavior may have been modified in this latest release compared to previous releases (this doesn't appear to happen in X3 and below), although that is not documented in the manuals.